The present invention relates to a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable.
The process and storage industries have long used various types of equipment to measure process parameters such as level, flow temperature, etc. A number of different techniques (such as mechanical, capacitance, ultrasonic, hydrostatic, etc.) provide measurement solutions for many applications. However, many other applications remain for which no available technology can provide a solution, or which cannot provide such a solution at a reasonable cost. For many applications that could benefit from a level measurement system, currently available level measurement systems are too expensive.
In certain applications, such as high volume petroleum storage, the value of the measured materials is high enough to justify high cost level measurement systems which are required for the extreme accuracy needed. Such expensive measurement systems can include a servo tank gauging system or a frequency modulated continuous wave radar system.
There are many applications that exist where the need to measure level of the product is high in order to maintain product quality, conserve resources, improve safety, etc. However, lower cost measurement systems are needed in order to allow a plant to instrument its measurements fully.
Further, there are certain process measurement applications that demand other than conventional measurement approaches. For example, applications demanding high temperature and high pressure capabilities during level measurements must typically rely on capacitance measurement. However, conventional capacitance measurement systems are vulnerable to errors induced by changing material characteristics. Further, the inherent nature of capacitance measurement techniques prevents the use of such capacitance level measurement techniques in vessels containing more than one fluid layer.
Ultrasonic time-of-flight technology has reduced concerns regarding level indications changing as material characteristics change. However, ultrasonic level measurement sensors cannot work under high temperatures, high pressures, or in vacuums. In addition, such ultrasonic sensors have a low tolerance for acoustic noise.
One technological approach to solving these problems is the use of guided wave pulses. These pulses are transmitted down a dual probe transmission line into the stored material, and are reflected from probe impedance changes which correlate with the fluid level. Process electronics then convert the time-of-flight signals into a meaningful fluid level reading. Conventional guided wave pulse techniques are very expensive due to the nature of equipment needed to produce high-quality, short pulses and to measure the time-of-flight for such short time events. Further, such probes are not a simple construction and are expensive to produce compared to simple capacitance level probes.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and to time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070. However, this new technology alone will not permit proliferation of level measurement technology into process and storage measurement applications. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher, and have an average power level of about 1 nanoWatt or lower. These factors present new problems that must be overcome to transmit the pulses down a probe and back and to process and interpret the returned pulses.
The present invention relates to a sensor apparatus for transmitting these low power, high frequency pulses down a probe and effecting their return. Currently, no industrially suitable sensor exists which can economically function as a transmission line and withstand typical industrial process and storage environments, while maintaining vessel integrity.
The present invention relates to a single conductor surface wave transmission line (Goubau line) adapted as a sensor for industrial process variable measurement. The present invention incorporates not only the transmission line function, but also a reference pulse means, a sensing function, a process connection mounting function, a sensor fixing means, and a process sealing means all in a single construction which is compatible with standard industrial mounting requirements such as flanges or threaded connections. The apparatus of the present invention also meets the heavy duty demands of an industrial environment and is suitable for installation in areas of high temperature, high humidity, high pressure, high chemical aggressiveness, high static electricity, and high electromagnetic influence. The sensor apparatus is connected electrically to a process measurement system electronics which provides its power and signal processing. The sensor apparatus is specifically designed to handle high speed, low power, high frequency broadband pulses which are delivered by the system electronics.
The sensor apparatus of the present invention is particularly adapted for the measurement of material levels in process vessels and storage vessels, but is not thereto limited. It is understood that the sensor apparatus may be used for measurement of other process variables such as flow, composition, dielectric constant, moisture content, etc. In the specification and claims, the term "vessel" refers to pipes, chutes, bins, tanks, reservoirs, or any other storage vessels. Such storage vessels may also include fuel tanks, and a host of automotive or vehicular fluid storage systems or reservoirs for engine oil, hydraulic fluids, brake fluids, wiper fluids, coolant, power steering fluid, transmission fluid, and fuel.
The present invention propagates electromagnetic energy down an inexpensive, single conductor transmission line as an alternative to conventional coax cable transmission lines. The Goubau line lends itself to applications for a level measurement sensor where an economical rod or cable probe (i.e., a one conductor instead of a twin or dual conductor approach) is desired. The single conductor approach enables not only taking advantage of new pulse generation and detection technologies, but also constructing probes in a manner similar to economical capacitance level probes.
As discussed above, the simplest implementations of a single transmission line in a process measurement probe will not withstand the previously discussed rigors of an industrial environment. Further, standard capacitance level probes do not accommodate the transmission of high speed pulses due to the electrical impedance discontinuities which exist in their assembly.
The present invention solves problems associated with implementing the new, inexpensive pulse technology by providing an improved mounting, fixing, securing, and sealing sensor apparatus including the combination probe element and transmission line. The present invention accomplishes these features while maintaining the electrical operation of a Goubau line including pulse launch, smooth impedance transition from cabling, reference pulse control, and transmission through the mounting including both transmitted pulse control and reflected pulse control.
According to one aspect of the invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into a vessel to measure a process parameter. The sensor apparatus includes a lower flange configured to be coupled to the vessel. The lower flange is formed to include a central aperture defined by a radially outwardly tapered surface located adjacent a top surface of the lower flange. The apparatus also includes a conductive probe element including a head having first and second radially outwardly tapered surfaces and an elongated conductive portion extending away from the head. The first tapered surface of the head is configured to engage the tapered surface of the lower flange to prevent movement of the probe element in a direction toward the lower flange. The apparatus further includes an upper flange configured to be coupled to the lower flange to secure the probe element to the lower flange. The upper flange includes a central aperture defined by a radially outwardly tapered surface located adjacent a bottom surface of the upper flange. The tapered surface of the upper flange is configured to engage the second tapered surface of the probe element to prevent movement of the probe element in a direction toward the upper flange. The apparatus still further includes a launch plate coupled to the upper flange, and an electrical connector coupled to the probe element. The connector is configured to couple the signal line to the probe element.
In the illustrated embodiment, the probe element is covered with an insulative material. The insulative material may have an increased thickness adjacent to the head of the probe element to improve sealing between the probe element and the upper and lower flanges. Illustratively, the probe element, the upper and lower flanges, and the launch plate are made from stainless steel.
In one illustrated embodiment, the tapered surface of the lower flange converges in a direction extending downwardly from the top surface of the lower flange, and the tapered surface of the upper flange converges in a direction extending upwardly from the bottom surface of the upper flange. The second tapered surface of the probe element is a divergent conical surface in a direction extending downwardly from a top end of the probe element, and the first tapered surface of the probe element is a convergent conical surface in a direction extending downwardly from the second tapered surface.
In another illustrated embodiment, the central aperture of the lower flange is formed to include a radially expanded cavity adjacent said top surface. The tapered surface of the lower flange is formed by a separate insert positioned within the cavity of the lower flange. The central aperture of the upper flange is formed to include a radially expanded cavity adjacent said bottom surface. The tapered surface of the upper flange is formed by a separate insert positioned within the cavity of the upper flange. The inserts are illustratively formed from a nonconductive material.
Also in the illustrated embodiment, the signal line includes signal conductor and a ground conductor. The electrical connector is configured to couple the signal conductor to the probe element and to couple the ground conductor to the launch plate. The launch plate and the upper flange are configured to generate a reflective reference pulse on the signal line as the electrical pulses move from the signal line to the probe element. In one illustrated embodiment, the apparatus further includes a static discharge resistor or complex impedance network coupled between the probe element and the launch plate.
According to another aspect of the present invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into a vessel to measure a process parameter. The sensor apparatus includes a lower flange configured to be coupled to the vessel. The lower flange is formed to include a central aperture extending between a top surface and a bottom surface of the lower flange. The apparatus also includes a conductive probe element extending through the central aperture of the lower flange, and an upper flange configured to be coupled to the lower flange to secure the probe element to the lower flange. The upper flange includes a central aperture extending between a top surface and a bottom surface of the upper flange for receiving a top end of the probe element. The apparatus further includes a first seal for sealing the probe and the lower flange, and a second seal for sealing the probe and the upper flange. The top surface of the lower flange is spaced apart from the bottom surface of the upper flange to permit any material passing through the first seal to escape therebetween. The apparatus still further includes a launch plate coupled to the upper flange, and an electrical connector coupled to the probe element. The connector is configured to couple the signal line to the probe element.
In one illustrated embodiment, the first seal is formed by a radially outwardly tapered surface located adjacent a top surface of the lower flange which engages a first radially outwardly tapered surface formed on the probe element. The second seal is formed by a radially outwardly tapered surface located adjacent a bottom surface of the upper flange which engages a second tapered surface of the probe element.
In another illustrated embodiment, the central aperture of the lower flange is formed to include a radially expanded cavity adjacent said top surface, and the tapered surface of the lower flange is formed by a separate lower insert positioned within the cavity of the lower flange. The central aperture of the upper flange is formed to include a radially expanded cavity adjacent said bottom surface, and the tapered surface of the upper flange is formed by a separate upper insert positioned within the cavity of the upper flange. The upper and lower flanges are illustratively made from a conductive material, and the upper and lower inserts are made from a nonconductive material.
According to yet another embodiment of the present invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into a vessel to measure a process parameter. The sensor apparatus includes a conductive probe element including a head having at least two different cross sectional dimensions along a length of the head, and a lower flange configured to be coupled to the vessel. The lower flange is formed to include a central aperture extending between a top surface and a bottom surface of the lower flange for receiving the probe element. The central aperture of the lower flange has at least two different cross sectional dimensions between the top and bottom surfaces of the lower flange to minimize impedance changes in the probe element adjacent the lower flange. The apparatus also includes an upper flange configured to be coupled to the lower flange to secure the probe element to the lower flange. The upper flange include a central aperture extending between a top surface and a bottom surface of the upper flange for receiving a top end of the probe element. The central aperture of the upper flange has at least two different cross sectional dimensions between the top and bottom surfaces of the upper flange to minimize impedance changes in the probe element adjacent the upper flange. The apparatus further includes a launch plate coupled to the upper flange, and an electrical connector coupled to the probe element. The connector is configured to couple the signal line to the probe element.
In one illustrated embodiment, the central aperture of the lower flange is defined by a radially outwardly tapered surface located adjacent a top surface of the lower flange, and the probe element includes a head having first and second radially outwardly tapered surfaces and an elongated conductive portion extending away from the head. The first tapered surface of the head being configured to engage the tapered surface of the lower flange to prevent movement of the probe element in a direction toward the lower flange. The central aperture of the upper flange is defined by a radially outwardly tapered surface located adjacent a bottom surface of the upper flange. The tapered surface of the upper flange is configured to engage the second tapered surface of the probe element to prevent movement of the probe element in a direction toward the upper flange.
In another illustrated embodiment, the central aperture of the lower flange is formed to include a radially expanded cavity adjacent said top surface, and the central aperture of the upper flange is formed to include a radially expanded cavity adjacent said bottom surface. If desired, a separate lower insert may be positioned within the cavity of the lower flange, and a separate upper insert may be positioned within the cavity of the upper flange. The upper and lower inserts are configured to engage the head of the probe element.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.